This is my understanding for your query. Precise & imprecise structures are completely based on specific industry practice. You can configure your structures in BOM as below . For EG

Parent Assm

- Sub Assm 1 ( Precise)

- Child 1/A

- Child 2/A

- Sub Assm 2 ( Imprecise)

- Child 3/B

- Child 3/B

Again, some of the organisation may prefer working with Imprecise (Sub assembly) structure prior to release. Once the entire components under the parent ( Root component) are ready to release, they may define the same structure as static ( Precise).